The present application claims priority to Japanese Applications No. P11-305518 filed Oct. 27, 1999, which application is incorporated herein by reference to the extent permitted by law.
1. Field of the Invention
This invention relates to nonvolatile memory and its driving method.
2. Description of the Related Art
Heretofore known as nonvolatile solid-state memory are flash memory and FeRAM, for example. Any of them requires three to four terminal for each information recording cell. In a structure connecting a number of terminals to each cell, the space for wirings degrades the integrating density. That is, in favor of increasing the integration density, it is necessary to enable reading of records by using simple cells with less terminals.
For the purpose of high-density recording of information, it is more advantageous to connect less terminals to each cell. For example, for accessing to solid-state memory, the minimum number of terminals is two.
However, conventional memory using two terminals involved not only the problem of disturbance in which writing into a selected cell undesirably influences non-selected cells but also the problem writing and reading could not stand together.
It is therefore an object of the invention to provide nonvolatile memory and its driving method capable of erasing, writing and reading records, simplifying the configuration of memory, and capable of high-density recording of information.
According to the first aspect of the invention, there is provided nonvolatile memory comprising a thin film phase change material having at least two stable phases, and a two-terminal switching element serially connected to the thin film phase change material.
In the first aspect of the invention, the thin film phase change material is typically configured to be changeable in phase when a voltage not lower than the threshold voltage of the two-terminal switching element is applied a current flows into the memory cell by applying a voltage not lower than the two-terminal switching element. This nonvolatile memory according to the first aspect of the invention can write information into the memory cell by changing the phase of the thin film phase change material.
In the first aspect of the invention, the thin film phase change material is typically configured to vary in electric resistance in response to its change in phase. Preferably, by supplying the memory cell with a voltage not lower than the threshold value of the two-terminal switching element, information can be read from the memory cell with reference to a difference in current flowing in the thin film phase change material.
According to the second aspect of the invention, there is provided nonvolatile memory comprising:
lower electrodes aligned in form of stripes; and
upper electrodes aligned in form of stripes such that lengthwise directions thereof intersect substantially at a right angle with lengthwise directions of said lower electrodes,
in portions between said upper electrodes and said lower electrodes where both electrodes intersect, memory cells each made of a thin film phase change material having at least two stable phases, a conductive adiabatic layer and a two-terminal switching element serially connected thereto.
In the second aspect of the invention, a plurality of layers of nonvolatile memory are preferably stacked, in which each nonvolatile memory has memory cells each including lower electrodes aligned in form of stripes, and upper electrodes aligned in form of stripes such that lengthwise directions thereof intersect substantially at a right angle with lengthwise directions of said lower electrodes, a voltage being applicable to said memory cells through said upper electrodes and said lower electrodes. Thus, in the nonvolatile memory having the multi-layered structure, the upper electrodes of the first nonvolatile memory are commonly used as the lower electrodes of the second nonvolatile memory stacked on the first nonvolatile memory.
In the first and second aspect of the invention, each two-terminal switching element is made of a pn junction or a np junction. Still in the first and second aspects of the invention, each two-terminal switching element typically has a pn junction or an np junction made of a p-type thin film semiconductor and an n-type thin film semiconductor. More specifically, the two-terminal switching element is an np junction or a pn junction made of a p-type thin film semiconductor and an n-type thin film semiconductor. In the first and second aspects of the invention, when an np junction is used as the two-terminal switching element and a reverse bias is applied to the np junction, the threshold voltage of the two-terminal switching element is a breakdown voltage.
In the first and second aspects of the invention, typically used as the two-terminal switching element is an element that decreases its electric resistance when a voltage above the threshold voltage is applied. In order to well control writing of information into the nonvolatile memory, an element that suddenly decreases its electric resistance upon application of a voltage above the threshold value is used as the two-terminal switching element.
According to the third aspect of the invention, there is provided a method for driving nonvolatile memory including lower electrodes aligned in form of stripes;
upper electrodes aligned in form of stripes such that lengthwise directions thereof intersect substantially at a right angle with lengthwise directions of said lower electrodes; and memory cells each made of a thin film phase change material having at least two stable phases, a conductive adiabatic layer and a two-terminal switching element serially connected thereto and located in one of portions between said upper electrodes and said lower electrodes where both electrodes intersect, characterized in that:
in a writing process, data is written in a selected memory cell by applying a voltage above the total value of the threshold voltage of each said two-terminal switching element and the voltage capable of changing the phase of said thin film phase change material between to one of said upper electrodes and one of said lower electrodes connected to the selected memory cell and thereby changing said thin film phase change material in phase, and
in a reading process, data is read out by applying a voltage not lower than said threshold voltage and lower than the total value of said threshold voltage and said voltage capable of changing the phase of said thin film phase change material, and thereby reading the current phase of said thin film phase change material.
Although Japanese Patent Publication No. 2743980 and Japanese Patent Publication No. 2806660 propose methods for easily writing information without using transistors. These publications, however, do not teach any method for reading and do not teach any method for preventing the problem of disturbance during writing or reading operation.
According to the nonvolatile memory according to the invention summarized above, since it includes memory cells each made of a thin film phase change material having at least two stable phases under the room temperature and a two-terminal switching element connected to the thin film phase change material, the current flowing in the thin film phase change material can be controlled by using the two-terminal switching element, and by using this current, it is possible to change the phase of the thin film phase change material and read out its current phase.
The above, and other, objects, features and advantage of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings.